Folding of Flexible Protein Fragments and Design of Nanoparticle-Based Artificial Antibody Targeting Lysozyme.

It is generally believed that a protein's sequence solely determines its native structure, but how the long- and short-range interactions jointly determine the native structure/conformation of the protein or every local fragment of the protein is still not fully understood. Since most protein fragments are unstructured on their own, direct observation of the folding of flexible protein fragments is very difficult. Interestingly, we show that it is possible to graft the complementary-determining regions (CDRs) of antibodies onto the surface of a gold nanoparticle (AuNP) to create AuNP-based artificial antibodies (denoted as Goldbodies), such as an antilysozyme Goldbody. Goldbodies can specifically recognize the corresponding antigens like the original natural antibodies do, but direct structural evidence for the refolding or restoration of native conformation of the grafted CDRs on AuNPs is still missing and in high demand. Herein we design a new Goldbody that targets an epitope on the lysozyme different from that of the previous antilysozyme Goldbody, and the one circle of helix in the CDR makes it possible to distinguish the unfolded conformation of the free CDR and its folded conformation on AuNPs by circular dichroism (CD) spectroscopy. The refolding of flexible protein fragments on NPs provides unique evidence and inspiration for understanding the fundamental principles of protein folding.